An unprecedented publishing event: to mark the centenary of its foundation, the British Security Service, MI5, has for the first time opened its archives to an independent historian. The book reveals the precise role of the Security Service in twentieth-century British history, from its foundation by Captain Kell of the British Army in October 1909, through two world wars, up to and including its present roles in counterespionage and counterterrorism. The book describes how MI5 has been managed, what its relationship has been with government, where it has triumphed, and where it has failed. In all of this no restriction has been placed on the judgments made by the author.

Defend the Realm also adds significantly to our knowledge of many celebrated events and notorious individuals and definitively lays to rest a number of persistent myths.Above all, it shows the place of this previously extremely secretive organization within the United Kingdom. Few books could make such an immediate and extraordinary increase to our understanding of British history over the past century.

“Absolutely fascinating…a sweeping and highly readable account of a century of British intelligence. –Washington Times

"This unique publication is definitive and fascinating. Definitive because, after decades of ill-informed or partial accounts this book fully defines and describes its subject; no future writer can ignore it. Fascinating because the fluent clarity of Andrew’s narrative, his eye for colourful individual detail and the sheer interest of his subjects…this book is essential reading for anyone with even the slightest interest in intelligence in the modern period." – Alan Judd, Spectator

"A scholarly and hugely entertaining account…often enthralling…Professor Andrew is an entertaining and authoritative guide through the labyrinth of secret files, with an infectious fascination for the game of counter-espionage…an important part of Andrew’s achievement is to narrate with clarity an incredibly complex story in which bizarre and improbable reality often outruns the most rococo fabrications of the spy novelist…the reader is left in no doubt that the defence of the realm is being vigorously conducted by the secret state with all the extraordinary powers at its command." – Robert McCrum, Observer

"MI5 is the first major security or intelligence service in the world to give a historian free range of its records—nearly 400,000 paper files, some with many volumes, say Christopher Andrew with a touch of exhaustion…it has been well worth the effort. The Defence of the Realm throws new light on an important area of the running of the country, analysing the changing threats to national security over the 100 years and discussing the appropriateness or otherwise of the service’s response. But just as interestingly, the book gets inside the culture of this secret service, showing how attitudes have changed with those changing threats; how woman have worked their way from the fringes to the heart of the organisation and how a sense of humour has always been important. It will be enthusiastically scrutinised by historians, intelligence buffs and conspiracy theorists…[there are] anecdotes and operational details as gripping as any thriller." – Stella Rimington, Financial Times

"Andrew’s magisterial study is an authorised, but not official, history and is clearly written, brilliantly organised and extremely readable, not least because of something he shares with many of MI5’s staff over the past hundred years—a sense of humour…" – Oleg Gordievsky, The Times

"To mark the centenary, the service took the unprecedented step of inviting Christopher Andrew, Cambridge historian and doyen of intelligence chroniclers, to write an authorised history. The outcome is weighty, measured and compelling…with this book, the author has done a formidably good job for both the service and the public interest…I find it hard to disbelieve much he asserts or denies…his narrative offers a feast for students and politics." – Max Hastings, Sunday Times

"A ripping read and just the kind of work one would hope for from a well-qualified academic who has been given the run of MI5’s treasure-trove of files. It is scrupulously documented, covering both the glory days of war, when MI5’s deception operations outsmarted Hitler, and the later nightmare penetrations by the double agent Kim Philby and friends, in which the KGB thoroughly outsmarted the British…Dramatically, it is confirmed that there was indeed a secret Wilson MI5 file, under the pseudonym ‘Worthington’…There is more." – David Leigh, Guardian

"The book covers everything from the agency’s origins 100 years ago as a shoestring outfit hunting German spies to the duping of the Nazis during the second world war, the scandals and successes of the intrigues against the Soviet Union and, latterly, the counter-terrorist campaigns first against the IRA and then against Jihadist suicide bombers…It is a striking experiment in openness." – Economist

"Professor Andrew’s account is magisterial, authoritative, balanced, readable and, particularly in the first half, full of wry humour and with an eye for the absurd. Andrew shows himself to be in command of his vast amount of material and be able to fashion it into a coherent narrative which is as comfortable with MI5’s organisational structure and development and relations with Whitehall as with individual cases and personalities…MI5 has been well-served by this history and so have future historians, Service staff and the public in general." – Andrew Lownie, Sunday Telegraph"The thousand pages of this book are brimming with some wonderful details…The Defence of the Realm is a valuable and important contribution to our understanding of the 20th century." – Susan Williams, Independent

"Andrew’s scholarship is meticulous and extensive. He deals at length with MI5’s role in the decolonization process in the Fifties and Sixties, something little known and still less appreciated. He also gives a superb account of the Service’s role in the struggle against IRA terrorism, casting new light on the Gibraltar shootings. There is a tantalizing section on MI5’s role in the battle against organized crime, especially drug trafficking; but that now seems to be taking second place to the Service’s counter-terrorism activities, in the wake of September 11 2001. MI5 could not have wanted a better historian than him. He has captured every important detail of the Service, but also its ethos and its place in our country as an institution. This book is unlikely to be surpassed for another 100 years, and until then will be the necessary starting point for anyone who wants to know what, exactly, MI5 is." – Simon Heffer, Telegraph